1. Field of the Invention
The present invention relates to air quality control method and apparatus for conditioning air on board aircraft and the like.
2. Description of the Prior Art
The prior art relating to internal aircraft conditions and the control thereof is well documented in aviation medical journals, aviation engineering journals, government aviation regulations and popular aviation magazines. Two publications of particular interest which discuss the basic principles and prior art in the field are "Fundamentals of Aircraft Environmental Control" by Alvin Ebeling (1968), Haiden Book Co., Inc., New York and "Aircraft Humidification System Development" by P. F. Halfpenny, Lockheed California Company. A major problem in cabin air quality control on board aircraft relates to the loss of moisture which escapes with air from inside the aircraft during high altitude flight. This problem is often compounded by the presence of an excess of ozone, pathogens, odors, and other contaminants which permeate the air on board the aircraft.
Prior to the advent of commercial jet aviation, the problem of retaining moisture on aircraft was less pronounced than it is today. Such aircraft flew at lower altitudes whereat the atmosphere has a higher moisture content.
In modern systems, the principal method of maintaining acceptable moisture levels is by means of an evaporation system whereby water is evaporated into the internal ambient air. Excess water is carried on board for this specific purpose. However, such systems have proved only marginally effective. Using such systems to provide about 30 to 50% relative humidity, requires large volumes of water. Furthermore, such water leaves salt residues upon evaporation, resulting in the introduction of dust into the cabin, thereby providing unsatisfactory results.
In another system presently in use, sprays of very fine water mist at ambient temperatures are injected into the cockpit and cabin area. In such systems the water source is limited to on-board supplies of potable water.
While the aircraft itself may be loaded with sufficient water to humidify the cabin air over the entire voyage, such a procedure would entail loading large amounts of water onto the aircraft, thus adding substantially to its weight. The additional weight of the aircraft quite obviously increases its fuel consumption and reduces available commercial space on the aircraft. Therefore, in the rare instances where aircraft are humidified by an evaporation system, no special water supplies are carried and portions of the drinking water are diverted for this purpose.
Since relatively little moisture is commonly injected into the low humidity air circulated throughout the aircraft in conventional systems, natural evaporation and expiration from the occupants contributes substantially to the on board relative humidity. With jet-propelled commercial aircraft commonly in use, flight durations may range from less than one and up to about 14 hours primarily at flight levels of 18,000 feet to 40,000 feet and in some instances as high as 50,000 feet. At these altitudes, the maximum possible moisture content in the ambient air is less than 1/20th that of the air at sea level. It can, therefore, be shown that approximately 95% of on board humidity on present day commercial aircraft takes the form of perpiration and expiration from the occupants themselves.
In order to provide some semblance of humidification on board aircraft, fresh air ventilation rates on board aircraft have been on the order of 15-20 cubic feet of fresh air per minute per person (cfmp). In some cases, the fresh air ventilation rates are even as low as 5 cubic feet per minute per person. By using such low fresh air ventilation rates, moisture generated by the occupants themselves is the major contributor to the humidification of the cabin air. Ventilation rates of 15-20 cfmp provide for less than a 10% relative humidity level. Thus, although fresh air ventilation rates in other enclosed areas such as restaurants and the like are on the order of 40 or 50 cubic feet per minute per person, the considerably lower ventilation rates on board commercial aircraft have been used as a compromise method for providing additional humidity.
In addition to providing for stale air conditions, such a low percentage of relative humidity is very unacceptable since the majority of travelers are accustomed to relative humidities on the order of approximately 50%.
Other factors prompting the recirculation of air within the aircraft rather than continuous ventilation with fresh air from exterior of the aircraft are the inherent ozone problems which occur, especially at higher altitudes and on most flights in the northern latitudes during winter and spring. Thus, when outside air is constantly drawn in, increased levels of ozone result on board the aircraft. Methods proposed for controlling the amount of ozone introduced into the aircraft have included charcoal filters. Such filters were investigated and subsequently abandoned owing to their inefficiency and unacceptable bulk. Most recently, a ceramic metal matrix catalytic converter installed in the high temperature, high pressure bleed air lines have been used. While such converters may prove useful at high temperatures and pressures, they are ineffective under less extreme conditions thus limiting the air intake to the bleed lines taken off of the turbine compressors.
In such systems, high air ventilation rates are very expensive since such rates are conventionally achieved at the expense of increased fuel consumption since the fresh air used is taken off of the turbine compressors.
Owing to the arid nature of the air on board the aircraft, a very uncomfortable situation developes. Not only is there considerable inconvenience caused through irritation of the mucous membranes, but such conditions also affect the skin and aggravate certain existing medical and health problems. Additionally, owing to the temporary, in some cases prolonged, dehydration and incapacitation of mucous membranes and other tissues and follicles, such conditions occasionally reduce resistance to harmful bacteria by people who would otherwise have had adequate defense mechanisms for defending against such microorganisms. Low relative humidity renders the defensive mechanisms of the body in the nasal passage and upper respiratory tract and the eye less effective in resisting infection.
Yet another problem, which would occur even if low fresh air ventilation rates were acceptable, results from the leakage which naturally occurs out of the cabin. Although ideally aircraft cabins should be sealed, leakage naturally occurs thereby further complicating the problem of aircraft humidification. Since it is impractical to recover moisture from uncontrolled aircraft leakage it must be considered that such moisture is effectively lost and must somehow be replenished if the aircraft occupants are to be subjected to an atmosphere having a comfortable relative humidity.
Finally, another disadvantage of prior systems operating with low fresh air-ventilation is the tendency of such systems to develop undesireably high concentrations of carbon dioxide within the cabin.
FIG. 1 describes the current state of the art wherein cold air 1 is fed into the aircraft structure 9. The cold dry air 1 is heated at 6 becoming warm dry air 3 and is then fed into the passenger cargo compartment 8 where it mixes with retained moisture or injected vapor 5 and becomes warm moist air, subsequently leaving the internal passenger cargo compartment 8 and the aircraft 9 as warm moist air. In such systems, the exhausted air is often recycled although not shown in the drawings.
In a similar prior art embodiment (not shown) water is brought on board the aircraft for the specific purpose of prolonging the retention of moisture on board the aircraft. In such an embodiment, cold dry air 1 is allowed to enter aircraft 9, the air becomes warm dry air 3 and is fed into the passenger cargo compartment 8 where it encounters water vapor 5 and becomes warm moist air 4. An additional heat source such as a boiler (not shown) may be used to vaporize the liquid stored on board so as to produce warm moist air 4. As was previously the case, the warm moist air 4 is exhausted from the internal passenger cargo compartment 8 and out of the aircraft 9 as warm moist air 4 while fresh dry air fed into the cabin must constantly be humidified with the limited available supply of water.
Because of relatively limited water supplies, the procedures described in the two previous embodiments lead to the problem of too much arid air in the passenger cargo compartment.